U.S. Pat. No. 4,553,829, issued Nov. 19, 1985 in the name of J. Bares, and entitled "Metering Blade for Use in a Development System" discloses a magnetic brush development station wherein developer material from a sump is provided to a magnetic brush for development of latent images on a photoconductor. A metering blade engageable with the brush has a plurality of apertures arranged in rows through which the developer material must pass as it travels from the sump to the magnetic brush for development of latent images. The size of the apertures controls the thickness of the layer of developer material advanced to the latent image. A similar disclosure appears in U.S. Pat. No. 4,523,833, issued June 18, 1985 in the name of T. B. Jones, and entitled "Developer Roller Metering Blade." In the Jones patent electrodes are connected to a voltage source and to the blade to regulate the quantity of developer material that passes through the apertures in the blade, thereby controlling the thickness of the layer of developer material on the magnetic brush. Column 6, lines 64-68 of the Jones patent suggests that adjustment of the intensity of the electrical field to a suitable level will prevent developer material from passing through the apertures so that an on/off type of control can be achieved.
Apparatus of the type disclosed in the patents identified above require a metering blade to be held tightly in engagement with the outer surface of the magnetic brush. If the shell of the brush rotates, substantial wear can occur. Even if the shell is stationary there is a critical adjustment of the blade against the shell in order to maintain the required control of developer material. If the material leaks under the blade contamination can result and complete shutoff of material will not be achieved. Moreover, the apparatus shown in both of such patents allows developer material to be in contact with the magnetic brush at all times, thus increasing the torque required to drive the magnetic brush even when no flow of developer material is desired.